1. Field of the Invention
The present invention relates to an optical subassembly, and more particularly to a combination of a semiconductor light emitting device and a plastic receptacle. The optical subassembly is used in the field of fiber optic communication, and the receptacle is combined with a fiber ferrule.
2. Description of the Prior Arts
Fiber optic communication is to transform the electrical signals emitted from the emitting end into optical signals, and then the optical signals are guided into an optical fiber and transmitted to the receiving end, after that the optical signals are transformed into electrical signals again. The emission and reception of the optical signals should be achieved by optical subassembly. For communication products, the quality requirement of the components is pretty high. The components need to be sealed with the method of hermetic sealing method, so as to prevent their operation from being damaged by outer atmospheric surrounding.
A conventional optical subassembly contains at least a diode on a header, a lens or a system of lenses, and a precise mechanical port for docking a fiber/ferrule assembly. It generally includes a receptacle and a light emitting/receiving optoelectronic device, wherein the receptacle is defined with a passage for insertion of a fiber ferrule. The light emitting/receiving device is employed to generate/receive optical signals. Namely, the optical subassembly in the system is also called the coupling unit. It can be concluded that an optical subassembly plays an important role in the optical communication. The optical power and light coupling efficiency provided by the optical subassembly decide the quality of signal transmission through a fiber. To improve the optical power and light coupling efficiency, the optical subassembly should be equipped with or formed with an optical beam transformation system, e.g., lenses, or it should be equipped with the light emitting device that can provide high output power.
Referring to FIG. 1, wherein an optical subassembly 10 includes a light emitting device 11 (ball-lens To-can) and a receptacle 12. The light emitting device 11 refers to laser diode or light emitting diode, and the receptacle 12 is made by plastic injection molding, along the axis of which is defined with a through passage 17 and an optical axis 18. The light emitting device 11 is combined with the receptacle 12 and an end of an optical fiber 13 is inserted in the passage 17 of the receptacle 12 and located opposite to the light emitting device 11. By such arrangements, after optical signals are emitted from the light emitting device 11, the optical signals can enter in the optical fiber 13. However, due to the divergence angle of the light emitted from the light emitting device 11, the light is unable to directly and totally enter in the optical fiber 13. In this case, a lens 14 can be disposed between the receptacle 12 and the light emitting device 11 in order to gather the light emitted by the light emitting device and guide it into the optical fiber 13. The lens 14 can be a spherical lens and disposed in alignment with the light emitting chip 16 on the top cap 15 of the light emitting device 11.
Since the spherical lens 14 is disposed on the top cap 15, there is assembly tolerance between the lens 14 and the light emitting chip 16, and it also has assembly tolerance when the light emitting device is assembled to the receptacle 12, thereby it is susceptible to the deviation of the lens 14 from the optical axis 18 of the receptacle 12, and some optical signals cannot be conducted to the optical fiber 13 by the lens 14. Moreover, the light coupling efficiency of the spherical lens 14 is low, thereby the operation of the optical subassembly 10 doesn't reach the optimum level. As concerns the light emitting device 11, the top cap 15 of which should be defined with a spherical lens 14, thus the cost is increased.
Referring to FIG. 2A, wherein a commonly-used conventional optical subassembly 20 includes a receptacle 22 and a light emitting device 21 (it is called flat-window TO-can). The receptacle 22 is defined with a passage 27 that has a close end. The close end of the passage 27 is defined with a lens 23, and an optical axis 28 is defined in the axial direction of the passage 27. The light emitting device 21 includes a metal cap 24 interiorly provided with a light emitting chip 25, the metal cap 24 is defined with a glass plate 26 on the top surface thereof. By such arrangements, the light emitted from the light emitting chip 25 can penetrate the glass plate 26 and then be gathered by the lens 23.
Since the glass plate 26 on the cap 24 has no light-gathering power, it doesn't matter whether the light emitting chip 25 is aligned to the cap 24 or not, the light emitting chip 25 can be aligned to the optical axis 28 by taking advantage of light coupling when assembling the light emitting device 21 to the receptacle 22, and the lens 23 is formed by plastic injection molding, such that the light coupling efficiency of the aspherical lens is effectively improved. However, since the light emitting device 21 should be equipped with the glass plate 26, the component cost of the optical subassembly 20 is increased.
On the other hand, when the lens 23 is located close to the light emitting chip 25, the light with a large divergence angle emitted by the light emitting chip 25 still can be projected in the scope of the lens 23. However, since the glass plate 26 is disposed on the cap 24 of the light emitting device 21, it limits the approaching distance between the lens 23 and the light emitting chip 25. Although the distance between the lens 23 and the light emitting chip 25 can be shortened by reducing the height of the cap 24, the light emitting device 21 has a certain regular specification as usual. If the height of the metal cap 24 is attributably changed and the quantity of the changed metal cap 24 is not large enough, the component cost of the optical subassembly 20 will also be increased. Vice versa, the light emitting chip 25 can be moved to be close to the lens 23 by increasing the height of the light emitting chip 25, but the production cost will be increased again.
The U.S. Pat. No. 5,973,862 discloses an optical subassembly, wherein the receptacle is provided with plural grip fingers for holding the spherical lens, however, it is unable to reduce the cost since the light emitting device is still provided with glass. On the other hand, the light coupling efficiency of the spherical lens is low, and the distance between the spherical lens and the light emitting chip in the light emitting device can not be shortened any more.
The U.S. Pat. No. 6,547,455 also discloses an optical subassembly, which uses a dual-lens device to increase the light coupling. However, since the light emitting device is provided with glass, the cost is relatively high, and the lens is unable to move closer to the light emitting chip in the light emitting device. Furthermore, installing the dual glass lens in the receptacle brings the possible shifting of the optical axial position, so that it may cause the misalignment with the light emitting device or the fiber. As a result, the structure with the dual lens in the receptacle is adverse to the light coupling effect.
The U.S. Pat. No. 6,283,644 discloses an optical subassembly, which has a dual-lens structure in a receptacle to increase the light coupling. However, the embodiment of the patent allows for passive alignment of the optical package, the coupling efficiency is adverseness.
The US 2005/0018981 A1 discloses an optical subassembly, which has a single lens in a receptacle, and the light emitting device is a TO-can with a glass. Thus, the cost of the light emitting device is expensive, and the lens fails to close to the chip which is disposed in the light emitting device.
The operation speed of the conventional optical subassembly of a LED module can reach 155 Mb/s at least, which can satisfy the requirement for the general users in local area network. The LED optical subassembly has the advantage of low cost in case of a large quantity of user terminals. However, the divergence angle of the LED device is rather large, only a lens is not enough to focus the light. In this case, the LED optical subassembly should be additionally equipped with a lens or lenses structure so as to improve the coupling effect. With reference to FIG. 2B, wherein the LED chip 110 includes an epitaxial portion 112 that is made of plural layers of semiconductor materials, including an active layer 114 that can generate light output by current injection. The epitaxial portion 112 is grown on a substrate 116, and the substrate 116 is provided with a monolithically integral micro lens 118. Since the substrate is almost transparent, the light emitted from the active region 114 will pass through the substrate and then to be focused by the micro lens 118, and thus the light coupling efficiency is accordingly improved.
However, to etch a micro lens 118 on the substrate 116, it should precisely control the etching uniformity on the whole wafer. In fact, the process is not easy to be controlled in production. As a result, the production cost is relatively increased. Furthermore, if the substrate 116 is thick, it will affect the light-gathered efficiency due to the light divergence. The substrate 116 has to be lapped to a desired thickness in order to improve the light-gathered efficiency. The thin substrate 116 is thus susceptible to be broken into pieces, leading to a reduced process yield.
On the other hand, since the light is emitted via the substrate 116, that is to say that the light is emitted from the backside of the chip, the on-wafer auto testing for the chips is difficult. The latter process is not easy to be controlled. Moreover, when passing through the chip 110, the light is initially focused by the micro lens 118 and then refocused by the lens of the receptacle. If the misalignment of the optical axis happens during the chip process, for example, if an error is appeared when making and aligning the micro lens on the substrate, the light coupling efficiency is drastically decreased. The transmitter optical subassembly comprised of the LED and the receptacle thus fails to meet the requirement of the specification and cannot be qualified for applications. The assembly of the monolithic integral lens of the LED and the single lens of package element can be provided enough output power in the conventional LED optical subassembly. But there are still some problems needed to be solved, such as manufacture inconvenience, high production cost, and power degradation by misalignment, and incompatibility with on-wafer testing, as mentioned above.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages of the conventional optical subassembly.